Fluorescent lamps are used in a wide variety of backlighting applications, such as for LCD displays, LCD televisions, and other types of consumer electronics. The fluorescent lamps are driven by an AC voltage. In mobile applications with typically only a DC power source, a DC/AC inverter is used to drive the fluorescent lamps. Even where AC power is available, a driver circuit is necessary to ensure that the appropriate AC driving waveform and voltage is applied to the lamps. The term controller encompasses both inverter and driver as used herein.
Typically, the backlight module includes more than one fluorescent lamp. When one or more of the fluorescent lamps fails, the failed fluorescent lamp presents an open circuit to the inverter or driver. This is referred to as an open lamp condition that causes the inverter to have an open lamp voltage.
Open lamp voltage handling and protection is often required in cold cathode fluorescent lamp (CCFL) inverter applications for safety and reliability reasons. In an open lamp condition, there might be a very large undesirable voltage occurring across outputs if protections are not in place. This undesirable voltage may be several times higher than a nominal output and could be harmful to circuit components. Thus, it is important for the inverter to safely and reliably operate under any anomalous conditions, such as an open lamp condition.
Under an open lamp condition, the lamp voltage will typically sweep up to 2˜2.5 times normal operating voltage. The controller will then try to strike the lamp for 1˜1.5 seconds. If the lamp does not turn on, the controller will shut down the system. Further, during the open lamp condition, the lamp voltage is much higher than the normal operating voltage. Therefore, the lamp voltage needs to be well controlled. If there are any instabilities in the system, the lamp and/or the controller can be easily damaged.
A prior art control scheme is shown in FIG. 1. The circuit includes a transformer with a primary side and a secondary side. The primary side is controlled by the (in this example) full bridge inverter. The full bridge inverter receives feedback from the secondary side of the transformer. Note that the lamp is connected to the secondary side of the transformer and sense nodes for current and voltage are used to feedback to the inverter.
Gvd is the transfer function from the duty cycle on the secondary winding to the output voltage. Gid is the transfer function from the duty cycle on the secondary winding to the lamp current. Cv represents the voltage sensing gain. Ri represents the lamp current sensing gain. Gm is the trans-conductance of the error amplifier. Gvt is the transfer function from the lamp voltage to time, during which the 140 uA current source discharges the compensation capacitor and lowers down the control voltage Vc. Fm represents the modulator gain.
Under normal operation, only the current loop operates. Under an open lamp condition, there are two cases. When all of the lamps are open, only the voltage loop works. When some lamps are open and some are still operating, both of the current loop and voltage loop work. Based on the system control chart in FIG. 1, the loop gain for both cases as shown in FIG. 2. FIG. 2(a) shows the loop gain under partial open lamp condition and FIG. 2(b) shows the loop gain under the complete open lamp condition.
The loop gain plots illustrate that the high Q of the resonant tank circuit of the inverter causes the system to be unstable because of little gain margin. It has also been found that a low frequency oscillation is observed. The frequency is determined by the difference between the resonant frequency and the switching frequency.
In order to achieve a stable system based on the prior art control method, either the loop gain must be lowered or the Q is dampened. If the gain is lowered, the lamp voltage regulation is lost. If the Q is dampened by inserting a resistor in the resonant tank, the system efficiency is adversely impacted.